The invention relates to a method for removing silicone oils from sludge wastes generated by a silicone manufacturing plant. In particular, the invention relates to a method for removing silicone oils and volatile organic compounds from waste water treatment plant sludge.
Removal processes for hydrocarbon oils from refinery sludge are known. For example, U.S. Pat. Nos. 5,288,391 and 5,690,811 disclose using a solvent from a refinery unit to extract oils from sludge. However, the oil being removed is typically hydrocarbon oil. Although the extraction process may work on silicone oil sludge, potential environmental liabilities may result. Further, using a solvent previously used at a silicones manufacturing plant is not believed to be possible because the solvent may already contain high levels of silicone oils and thus may not extract oils. Therefore, fresh solvent would be needed for extraction.
Another extraction process is set forth in U.S. Pat. No. 5,271,851, which discloses a process for filtering sludge, washing the sludge with a solvent, and steam stripping the sludge to remove petroleum oils. In another process, an aerobic biomass can be used to degrade the hydrocarbon oils, as disclosed in U.S. Pat. No. 5,207,912. However, silicone oils are much more difficult to degrade. U.S. Pat. No. 5,269,906 discloses an extraction process that involves centrifuging an oily sludge stream with about 5% solids to remove the water and free oil. This step is followed by heating, for example, by indirect heat exchange to remove all of the water and oils resulting in typically bone-dry solids. This process attempts to recover all oil for reuse, and does not focus on the state of the solids remaining.
Thus, a need exists for the treatment of the solids that remain after recovery and extraction processing.
It is desirable to utilize a method for removing silicone oils from waste water treatment plant (WWTP) sludge in a silicones manufacturing plant, such that the resulting sludge cake can be a handleable, non-dusty sludge cake. It is also desirable that the process removes a significant amount of volatile organic compounds (VOCs) from the sludge.
One aspect of the present invention is to produce an acceptable sludge cake having a low oil concentration and at least 10% water in order to avoid a dust problem when handling the sludge. If the sludge laden with silicone oils is dried completely, some high boiling silicone oils still remain on the solids, making the resulting solids water resistant and difficult to rewet. Additionally, the solids are fine and will dust when dried.
Another aspect of the present invention is to remove not only silicone oils from the sludge, but also to remove volatile organic compounds (VOCs) that are commonly found in the waste water of silicones manufacturing plants.
As embodied by the invention, silicone oils can be removed from process sewer sludge through a specific process sequence of first adding biosolids to the waste water at low pH to reduce the biosludge volume and increase the solids to oil ratio; centrifuging the waste water to dewater the sludge and concentrating the silicone oil contaminated solids, while removing a fraction of the silicone oils from the solids which allows a higher oil removal before the cake becomes too dry; and finally removing more silicone oils from the solids through a drying process. The heating process must be controlled and mixing must be involved to ensure that spot drying does not occur. The silicone oils will not be recovered for reuse.